The field of process chromatography is concerned with analyzing gas samples flowing through a process pipeline. A sample from a gas pipeline may be taken by use of a sample probe or other sampling device, which then provides the sample to a gas chromatograph. The gas chromatograph separates the sample into its individual components, using a variety of detectors to analyze the concentration of the resulting component bands in the sample. In the oil and gas industry the knowledge of what fluid is being transported by the pipeline is useful for a variety of purposes, such as source identification and custody transfer.
FIG. 1 shows a known gas chromatograph system (not to scale). Gas flows through a process pipeline 110, a sample of which is taken by a sample probe 120 prior to being introduced to gas chromatograph (GC) 100. The gas sample may be filtered and heat traced generally along tubing 130 before flowing into gas chromatograph 100. Heating may be required for gases that may condense into a part gas, part liquid flow at cooler temperatures. After being analyzed by the gas chromatograph, the gas sample is either returned into the process pipeline 110, or vented to the atmosphere. As used herein, the term gas chromatograph is being used in its broad sense, to include what is traditionally known both as the sample handling system and as the carrier pre-heat system.
Referring to FIG. 2, gas chromatograph 100 includes valve assembly 210 connected to multiple columns 220 and detectors 230, in this case, thermal conductivity detectors (TCD""s). A gas sample generally follows path 240 through valve assembly 210, columns 220 and TCD""s 230. The valve assembly allows the selection of columns 220 which contain a liquid phase, or porous polymer, or other material. Two types of columns are: 1) packed columns, filled with a liquid coated solid support or porous polymer; and 2) capillary columns, coated with a liquid or porous polymer. In either case, these materials act to separate the gas sample into multiple fractions, each fraction that is to be analyzed being sequentially directed to the TCDs 230. For example, a gas sample may contain various molecular weight hydrocarbon components such as ethane, methane, and heavier molecules. Ideally, each of these components would be analyzed individually. The resulting analysis could be normalized to minimize the effects of varying sample size from one injection to the next. In general, column 220 separates the gas sample so that more volatile components would elute from the column first, followed by less volatile components (although the use of valve switching may cause the components not to elute at the detector in that order).
Referring to FIGS. 3A and 3B, the operation of a sample valve is shown. Valve 300 includes a plurality of valve ports, labeled 1-6. Incoming line 310 provides a gas sample to valve 300. Exhaust line 320 expels the gas sample from the valve 300. Solid lines 330 show open passages between ports, whereas dotted lines 340 indicate blocked passages between the ports.
A solenoid (not shown) places valve 300 into either an ON position, as shown in FIG. 3A, or an OFF position, as shown in FIG. 3B. When a valve is in the ON position, sample gas flows from incoming line 310, through port 1 to port 6, through line 315 and finally through port 3 to port 2 and out exhaust line 320. When the valve is in the OFF position, sample gas flows from incoming line 310, through port 1 to port 2 and out through exhaust line 320. At the same time, carrier gas flows through port 5 to port 6 into line 315 where it displaces the sample gas. The carrier gas then flows from port 3 to port 4 and injects the sample onto the column. Of course, the designation of OFF versus ON is somewhat arbitrary and the opposite nomenclature could also be used.
FIG. 3C illustrates how a pair of valves may operate either alone or in combination with additional valves (not shown). A first valve 300 includes an array of six valve ports. A second valve 350 also includes an array of six valve ports. Associated tubing 310, 315, 320, 325 and 390, and columns 360 and 370 are also shown as well as dual TCD""s 380.
Incoming line 310 is attached to a sample transport line (not shown). When first valve 300 is in an OFF position, gas sample flows from incoming line 310 to port 1 to port 2 of the valve 300 and out exhaust line 320. When valve 300 is in an ON position, however, gas sample flows from port 1 to port 6 and then through sample loop 315. That gas then flows from port 3 to port 2 of valve 300 and is expelled out exhaust line 320. At this time, the sample loop 315 is filled with a gas sample. This means that, if valve 300 is turned OFF at this time, a gas sample is trapped within the sample loop 315.
Turning now to valve 350, when it is in an OFF configuration, carrier gas flows from carrier gas input line 390 through port 2 of valve 350, to port 1 and then through carrier tubing 325. At this time, valve 300 is also in an OFF configuration, so that the carrier gas in tubing 325 is forced through port 5 to port 6 and through gas sample tubing 315. Consequently, this action forces the gas sample down column 360 via ports 3 and 4. The gas sample can then additionally be forced through column 370 and into the dual TCD 380 via ports 4 and 3. Thus, the valves may be connected in series to form xe2x80x9cchannels.xe2x80x9d Each channel feeds into a corresponding thermistor pair (a measurement thermistor and a reference thermistor), which measures the amount of a component in the process sample. Alternatively, downstream analyzer valves can be arranged in the system to select a desired column or detector. The graph on which the data are presented has a series of peaks corresponding to the detected components (such as ethane, methane, etc.), and is generally referred to as a chromatogram.
FIG. 7 shows an example of a chromatogram. As various molecules elute from the columns 460 based upon their volatility, they are measured by a concentration-dependent detector such as a thermal conductivity detector (TCD), a flame photometric detector (FPD), a photoionization detector (PID), a helium ionization detector (HID), or an electrolytic detector. The measured values appear on the chromatogram as a series of peaks. The peak maximum corresponds to the absolute retention time (i.e. time elapsed from injection of sample) for each component in the gas chromatograph system, with the area under each peak being related to the concentration of that component in the sample. To operate the system most efficiently, the valve switching directs the samples from column to column at predetermined times. The columns are sized to provide adequate time between critical components (i.e. for valve switches).
FIG. 4 illustrates a simplified gas chromatograph 400 as is broadly known in the art. Sample valve 410 connects to sample-in line 420, sample out line 430, carrier-in line 440 and column line 450. Sample-in line 420 connects to sample shut-off valve 470 upstream of the sample valve 410. Immediately upstream of sample shut off, sample in line 420 connects to a sample pre-heat coil. Further upstream, sample-in line 420 connects to, e.g., a process pipeline (not shown). Downstream of the sample valve 410, column line 450 connects to column 460. Column 460, in turn, connects downstream to the remainder of the gas chromatograph, including TCD 480, with measurement line 481 and reference line 482.
During operation, a sample of fluid is delivered from a process pipeline or similar source through sample-in line 420. Once the sample is inside the sample valve 410, sample shut off valve 470 is actuated, closing off sample valve 410 from the upstream sample source. At this time, the sample in the sample valve 410 is allowed to equilibrate with atmospheric pressure by exhausting or bleeding the excess sample through sample out line 430. At this time the sample valve 410 is actuated, changing the internal flow of the sample valve 410. Carrier-in line 440, holding pressurized carrier gas, such as helium, hydrogen, nitrogen or argon, is now in communication with the sample trapped in the sample valve 410. This carrier gas displaces the sample out column line 450 and to column 460.
One problem with the arrangement of FIG. 4, however, is the temperature variation of inlet sample gas. Variations in temperature between samples of fluid affect the amount of sample (i.e. number of moles) held in sample valve 410, and therefore carried to column 460. This affects the accuracy of the measurements downstream at the TCD""s (or other detectors).
More specifically, from the Ideal Gas Law, it is known that:
PV=nRTxe2x80x83xe2x80x83(1)
Where,
P=pressure;
V=volume;
n=number of moles;
R=gas constant; and
T=temperature.
Due to a fixed-sized sample loop, the sample volume inside the sample valve 410 is essentially constant. Therefore, a first problem with known gas chromatographs is that the number of moles in the sample injection varies directly with sample pressure and inversely with sample temperature. Variations in temperature or pressure therefore change the number of moles in the sample, and this change in the number of moles impacts the reproducibility and analytical precision of the gas chromatograph. Consistent sample injections are especially important for chromatography applications that can""t normalize the data, such as heartcut or backflushing of part of the sample to vent.
A second problem is xe2x80x9cretention time driftxe2x80x9d that arises from differences in temperature between the inlet sample and the carrier gas. FIG. 9 shows an example of retention time drift when the inlet sample temperature or carrier temperature is warmer than the column temperature. This is a problem because where the component peaks overlap or extend beyond the switching time for a corresponding analyzer valve, the offending portion of the curve is not measured by the chromatograph.
In process chromatography, it is important to have short analysis times to provide sufficient analytical feedback for process control. For this reason, the process chromatographer sets the switching times as close together as realistically possible to provide the fastest possible chromatograph, and so merely allowing more component separation (i.e. longer analysis times) is not a best-case solution.
It is desirable, therefore, to heat the inlet sample and carrier gas to the gas chromatograph temperature, usually chosen in the range of 80-85xc2x0 C. with little variation. It has been difficult to heat the inlet gas to a consistent temperature, however. One effort involved placing a length of tubing inside a heated zone, while at the same time, coiling the tubing in a compressed corkscrew manner to conserve space. However, even heating of very long coils of tubing, such as 50-foot coils, does not reliably heat the inlet gas to the desired temperature. This is due to the fact that the ambient temperature of a process gas chromatograph varies from xe2x88x9218 to 55xc2x0 C. Further, this approach is a less than ideal method of heating the inlet gas because the extra length of tubing results in additional costs, spatial requirements, and complexity when designing a heated zone for the gas chromatograph.
A related problem is variation in component retention time arising from fluctuations in the inlet carrier pressure. Since inlet pressure fluctuations affect the carrier flow rate, they also result in retention time drift. It is desirable therefore to eliminate or minimize these variations in inlet carrier pressure.
A third significant problem is that of xe2x80x9cbaseline drift.xe2x80x9d FIG. 8 shows the effects of baseline drift on a simplified chromatogram. The drift has been exaggerated to illustrate the measurement error. The curve produced by the measurement element, such as a TCD, is based upon the actual baseline. However, the actual baseline has xe2x80x9cdriftedxe2x80x9d or dropped below the assumed baseline. Because the peak integration algorithms make certain assumptions regarding the area underneath the curves, including determining the assumed baseline, an error is introduced by baseline drift. In particular, the peak integration algorithms fail to detect any portion of the curve that falls above the actual baseline yet below the assumed baseline.
Baseline drift occurs where there is a temperature difference between measurement and reference thermistors, filaments or other detector elements. Referring again to FIG. 4, TCD 480 includes measurement line 481 and reference line 482. A thermal conductivity detector operates based on measuring the thermal conductivity of the fluid at the measurement point as compared to the fluid at the reference point. Thus, the inlet carrier gas temperature can affect the measurement stability of the thermal conductivity detector and any fluctuation in temperature of the reference relative to measurement results in detector baseline drift. However, although it is therefore important that the fluid flowing through the measurement and reference lines are at the same temperature, variations are common due to the fact that the ambient temperature of a process gas chromatograph varies from xe2x88x9218 to 55xc2x0 C. As previously described, the preheat coils for the referenced inlet carrier gas are unable to achieve the desired temperature.
A fourth significant problem is that of xe2x80x9cband spreadingxe2x80x9d. Unlike retention time drift, where the entire curve shifts to one side or another, band spreading involves the widening of the entire band curve. FIG. 11 (not to scale) shows the effects of band spreading on a simplified chromatogram. Curve 1101 is a chromatogram curve without band spreading, while curve 1102 is the corresponding curve with band spreading.
As can be appreciated, a great amount of information can be determined from an accurate chromatogram curve. Referring still to FIG. 11, in the Figure the term t represents time, tr is retention time, h is height, Wb indicates the width at the base of the curve, W0.5 represents the width of the curve at the half-height, Wi is the width of the curve at the inflection point, and 0.607 h shows the height of the curve at the inflection point. With band spreading, it is more difficult to identify these points accurately. Further, if the band curve becomes spread beyond the desired switching time, a portion of the curve would not be measured by the chromatograph. Alternately, the valve switching time could be delayed for the elution of the component but this would lead to longer analysis times. As mentioned previously, it is important to have short analysis times in process chromatography to provide good process control. Thus, excessive band spreading results in measurement errors or longer analysis times.
The problem of band spreading arises from gas decompression and carrier velocity acceleration as the sample travels through the column. As a result, most of the separation of components completes at the front of the column. Historically, chromatograph research has focused on developing small diameter capillary columns to compensate for this problem. However, this solution has been unsatisfactory because the complexity of the gas chromatograph varies directly with column diameter and the reliabilty varies inversely. Gas chromatographs with very small column diameter (i.e.  less than 0.25 mm ID) are impractical for process (on-line) applications.
As can be seen, a number of problems exist with current gas chromatographs and a gas chromatograph is needed that solves these and other problems. The ideal process gas chromatograph would be both fast and accurate, eliminating or severely reducing many of the measurement errors known in the prior art. It would also be simple and inexpensive to manufacture. In a perfect world, the device or method that solves these problems would do so on its own, requiring little human supervision or maintenance. It would also have considerable longevity, including being sturdy and not prone to breakage.
A first embodiment of the invention is a sample handling system for a gas chromatograph, including a back pressure restrictor for the transport of a constant flow rate of fluid sample, a valve attached to the downstream end of the back pressure restrictor, and a separation column attached to the valve for eluting the sample into component parts. Preferably, the backpressure restrictor is capillary tubing. Ideally, the ratio of the fluid outlet pressure of the capillary tubing to the inlet pressure of the capillary tubing should be maintained at less than 0.528 to attain critical (i.e. laminar) flow. Although critical flow provides the maximum benefit, a pressure ratio approaching 0.528 from above would provide some benefit. Even more preferably, the sample handling system includes an insulated region having a heater, with the sample of pre-heat tubing being inside the insulated region and being upstream of the capillary tubing. The valve in the system may be an initial sample valve in the flow of the fluid sample through the system.
A second embodiment of the invention is a gas chromatograph including a measurement element with reference and component measurement locations, a transport line carrying carrier fluid and attached to the reference measurement location, carrier fluid pre-heat tubing connected to the transport line to warm the carrier fluid, and a back pressure restrictor connected to, and downstream of, the carrier fluid pre-heat tubing. Ideally, the backpressure restrictor is capillary tubing having an outlet to inlet pressure ratio of less than 0.528. Again, a pressure ratio approaching 0.528 from above would provide some benefit.
A third embodiment of the invention is a sample handling system including a separation column to separate a fluid sample into component parts, a measurement device downstream of the column, and a back pressure restrictor between the column and the measurement device. Preferably, the restrictor is capillary tubing.